Hitherto, a method has been known as the color or monochromatic image forming techniques, in which an ink sheet containing a thermal diffusible dye capable of diffusing and transferring by heating is faced to an image receiving layer of a image receiving sheet and the image diffusible dye is imagewise transferred to-the image receiving layer using a heat printing means such as a thermal head or a laser to form an image, so called as a dye thermal transfer method. Such the thermal transfer method is known as a method by which the image formation employing digital data is made possible and a high quality image being a much for that of silver salt photographic image can be obtained without the use of any developer.
However, the image obtained by such the method has a drawback such as that the image is inferior in the storage ability and the durability of the image to that of the silver salt photographic image.
A thermal transfer material and an image forming method using it, hereinafter referred to as a post-chelating method, employing a thermal diffusible dye capable of forming a chelate, hereinafter referred to as a post-chelate dye, have been proposed to improve the stability of the image, particularly, the fixing ability and the light fastness of it, as is shown in, for example, Japanese Patent Document Open to Public Inspection, hereinafter referred as Japanese Patent O.P.I. Publication, Nos. 59-78893, 59-109349 and 60-2398.
Moreover,. Japanese Patent O.P.I. Publication No. 4-89292 proposes a method in which the dye is further subjected to heat after the transfer for improve the storage ability of the image obtained by the post-chelating method.
As the method for improving the mechanical durability (resistivity against wearing and skin oil contamination) of the image formed by the thermal dye transfer method, a method has been known in which a transparent protective layer is formed on the image by thermal transfer process, and a method for forming a transparent protective layer by the thermal transfer process on the image formed by the post-chelating method are disclosed.
Moreover, a method for improving the storage ability of the image obtained by the post-chelating method by addition of a metal ion-containing compound into the transparent protective layer for accelerating the chelating reaction of the unreacted dye remaining just after the printing is disclosed, for example, in Patent Documents 1 and 2.
As one of the methods for forming the transparent protective layer on the image formed by the thermal dye transfer method, a constitution in which an UV absorbent is contained in the transparent protective layer is disclosed, for example, in Patent Documents 3 through 5. The method of employing the UV absorbent regarding to the thermal dye transfer method employing the post-chelating technique is also described-in the foregoing Patent Documents 1 and 2. However, when the transparent protective layer contains both of the metal ion compound and the known UV absorbent in the thermal dye transfer method utilizing the post-chelating method, a problem is raised-that the whiteness of the background of the whole image is degraded by coloring caused by the reaction of the metal ion compound and the UV absorbent by heating on the occasion of the transferring of the transparent protective layer after the image formation. Furthermore, it is a drawback that the transparent protective layer before the transference itself is colored during the storage for long period.                Patent Document 1; Japanese Patent O.P.I. Publication No. 5-42774        Patent-Document 2; Japanese Patent O.P.I. Publication No. 2000-168244        Patent Document 3; Japanese Patent Examined Publication No. 2925699        Patent Document 4; Japanese Patent Examined Publication No. 3395090        Patent Document 5; Japanese Patent O.P.I. Publication No. 7-276831        